1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device, and more particularly, relates to a method for manufacturing a semiconductor device that has a thin film transistor (hereinafter, abbreviated as “TFT” in this specification) formed with the use of a semiconductor film, where the semiconductor film is formed due to lateral crystal growth of a semiconductor film formed over an insulating substrate by laser light irradiation.
2. Description of the Related Art
Recently, a technique of crystallizing a semiconductor film (particularly, an amorphous semiconductor film) formed over an insulating substrate such as glass or improving the crystallinity of the semiconductor film by laser annealing has been widely studied. For the semiconductor film, silicon is typically used.
A glass substrate has more workability as compared with a synthesized quartz glass substrate that is often used conventionally, and has the advantage of being able to manufacture a large-area substrate. However, a glass substrate has a lower melting point as compared with a synthesized quartz glass substrate, and thus has the disadvantage that heating is not able to be performed so much in a process of crystallizing a semiconductor film formed over the glass substrate. Consequently, a method using a laser is often used as a method for crystallizing a semiconductor film formed over a glass substrate. This is because high energy can be given to only an amorphous semiconductor film by the laser without increasing the temperature of a substrate so much.
A crystalline semiconductor film formed by laser annealing has a high mobility. Therefore, TFTs formed with the use of this crystalline semiconductor film are extensively used for a monolithic liquid-crystal electro-optical device in which TFTs of a driver circuit and a pixel portion are formed over the same substrate, and the like.
Further, a method is industrially excellent in productivity, and thus is widely used. In the method, a laser beam by pulsed oscillation that is powerful such as an excimer laser is processed by an optical system to be a several-centimeter square spot or a linear shape 10 cm or more in length at a surface to be irradiated, and laser annealing is performed while scanning the laser beam relatively with respect to the surface to be irradiated.
In particular, when a linear beam is used, the productivity is high since the whole of a surface to be irradiated can be irradiated with the laser beam by scanning only in a direction perpendicular to the longitudinal direction of the linear beam unlike a case of using a spot-shaped laser beam and thus requiring scanning from front to back and from side to side. The reason of the scanning in the direction perpendicular to the longitudinal direction is that the direction is the most efficient scanning direction. Due to this high productivity, the use of a linear beam obtained by processing a laser beam of a pulsed-oscillation excimer laser by an appropriate optical system in laser annealing is now becoming mainstream in manufacturing technology of a device such as a liquid crystal display device using a TFT. Further, this technique makes it possible to manufacture a monolithic liquid crystal display device in which TFTs of a driver circuit provided at the periphery of a pixel portion are integrally formed.
However, a crystalline semiconductor film that is manufactured by laser annealing is formed of an aggregate of a plurality of crystal grains, and the crystal grains are random in position and size. A TFT that is manufactured over a glass substrate is formed by separating the crystalline semiconductor film into island-shaped patterns for element separation. However, in this case, it is not possible to specify the positions and sizes of the crystal grains to form the TFT. There are an infinite number of recombination centers and trapping centers derived from amorphous structures and crystal defects at interfaces of the crystal grains (grain boundaries) as compared with within the crystal grains. When a carrier is trapped in this trapping center, the current transporting property of the carrier is known to be decreased since the potential of the grain boundary is increased to become a barrier against the carrier. While the crystallinity of a semiconductor film of a channel forming region has an important effect on electrical characteristics of a TFT, it is almost impossible to form the channel forming region with the use of a single-crystal semiconductor film by eliminating effects of the grain boundaries.
In order to solve these problems, there have been various attempts to form crystal grains that are subjected to position control and are large in grain size in laser annealing. The solidification process of a semiconductor film after irradiating the semiconductor film with a laser beam will be described below.
It takes some time to generate a crystal nucleus in the semiconductor film melted completely by laser beam irradiation, an infinite number of crystal nucleuses are generated uniformly (or non-uniformly) in a completely melted region of the semiconductor film, and crystal growth is terminated to complete the solidification process of the completely melted semiconductor film. The crystal grains obtained in this case are random in position and size.
Alternatively, when the semiconductor film is not completely melted by laser beam irradiation so that a solid-phase semiconductor region partly remains, crystal growth proceeds from the solid-phase semiconductor region after laser beam irradiation. As previously mentioned, it takes some time to generate a crystal nucleus in the completely melted semiconductor film. Therefore, until a crystal nucleus is generated in the completely melted semiconductor film, a solid-liquid interface that is a head of crystal growth (which indicates a boundary between the solid-phase semiconductor region and the completely melted region) moves in a parallel direction (hereinafter, referred to as “a lateral direction”) with respect to the surface of semiconductor film so that a crystal grain grows to many times its film thickness. Such growth is completed in such a way that an infinite number of crystal nucleuses are generated uniformly (or non-uniformly) in a completely melted region and crystal growth is terminated. Hereinafter, this phenomenon is referred to as super lateral growth.
In the case of an amorphous semiconductor film or a polycrystalline semiconductor film, there is an energy region of a laser beam for achieving the super lateral growth. However, the energy region is quite narrow, and it is not possible to control positions in which crystal grains that are large in grain size are obtained. Further, a region other than the crystal grains that are large in gain size is a microcrystalline region in which an infinite number of crystal nucleuses are generated or an amorphous region.
As described above, when the temperature gradient in a lateral direction can be controlled (a heat flow in the lateral direction can be generated) in an energy region of a laser beam for completely melting a semiconductor film, the position and direction in growth of a crystal grain can be controlled. In order to accomplish this method, there have been various attempts.
First, as a reflective film, a metal film (a Cr single layer or a lamination layer of a Cr film and an Al film stacked on the Cr film) is formed on an amorphous semiconductor film, and is partially etched to form a region with the metal film on the amorphous semiconductor film and a region without the metal film. At a wavelength of 308 nm, the reflectivity of Cr is approximately 60%, and the reflectivity of Al is approximately 90%. Therefore, when a laser beam with a wavelength of 308 nm is used to irradiate the amorphous semiconductor film, the amorphous semiconductor region below the metal film is less irradiated with the laser beam as compared with the amorphous semiconductor covered with no metal film. Namely, a temperature gradient is generated between the amorphous semiconductor region below the metal film and the amorphous semiconductor covered with no metal film. Therefore, a crystal nucleus generated in the amorphous semiconductor region below the metal film is known to laterally grow to the amorphous semiconductor covered with no metal film, which is kept in a melted state, and to form a crystal grain of 1 to 2 μm.
However, this method has the following problems. In the method of performing crystallization by forming the metal film partially on the amorphous semiconductor film and performing laser beam irradiation, it is difficult to control the position of a crystal grain on a single crystal basis while a position in which a crystal grain is formed can be controlled. In addition, since the metal film is formed directly on the amorphous semiconductor film, the metal element diffuses into the amorphous semiconductor film, and there is a possibility that deterioration in electrical characteristics of a TFT is caused when the TFT is manufactured with the use of a crystalline semiconductor manufactured by crystallizing the amorphous semiconductor film. In addition, there is a possibility that cracks and peeling is generated in the metal film and the amorphous semiconductor film. The metal film is typically formed by sputtering. As compared with CVD, sputtering shows larger in-plane variations in film thickness when the metal film is formed. Therefore, when substrates are larger, it can be said that the use of sputtering is not preferable in the feature.
Further, James S. Im et al. show a Sequential Lateral Solidification method (hereinafter, referred to as “an SLS method”) by which super lateral growth can be achieved in any position (for example, refer to Patent Reference 1).                (Patent Reference 1) Japanese Patent No. 3204986        
In the SLS method, crystallization is performed while a mask in the shape of a slit is moved on the order of the distance of super lateral growth (approximately 0.75 μm) for every shot.
The use of the SLS method makes lateral growth possible by irradiating an amorphous silicon film formed on a glass substrate with an extra fine beam on the order several microns, where the extra fine beam is obtained by condensing an excimer laser beam passed through a slit. In addition, lateral growth can be proceed continuously by controlling the substrate feed pitch for every shot to be the distance of lateral growth. However, the use of the SLS method has the following problems.
Since an excimer laser is not so good in quality, it is necessary that a mask for condensing light into several μm be used in a laser irradiation system in order to use the SLS method. Further, the mask needs to be replaced periodically, and an automatic focus function for keeping a focal depth constant is indispensable in order to keep a focal point uniformly in the surface of a substrate since a lens that has a short focal length also has a shallow focal depth. As a result, the system becomes complicated and expensive as compared with a typical laser irradiation system.